Plasma dry etch machines on reactors are vacuum chambers in which an electrical plasma is created in order to etch semiconductor wafers. The etching is usually performed through a photoresist mask. Quite often it is desired to etch oxide layers preferentially over adjacent polysilicon such as the wafer substrate.
The dry etching of certain films (e.g., silicon dioxide) used in semiconductor manufacturing involves the use of fluorocarbon containing gases to effect good selectivity to the silicon or silicon nitride substrates typically used in the fabrication process. One of the most notable examples of a dry etch is the high aspect silicon dioxide contact etch.
In the dry etch process, a wafer is placed over a metallic cathode plate and plasma is discharged to flow from a cathode to the anode plate, where the wafer functions as the cathode.
During the processing of high aspect contacts, a CF.sub.4 +CHF.sub.3 chemistry is used to effect a high etch rate, while at the same time maintaining sufficient selectivity to the silicon substrate. The contact etch is achieved by the selective deposition of the carbon/fluoride based fluorocarbon polymers onto surfaces which are not to be etched. As a result of this process, the fluorocarbon polymers become deposited on grounded portions of the reactor surface. Similarly, in the other etching processes, a fluorocarbon polymer is applied to the wafer in order to cause the preferential etching of the silicon dioxide over silicon. For example, etching of silicon dioxide in a planar system takes place in C.sub.2 F.sub.6, which provides good selectivity when etching silicon dioxide over silicon substrates. Selectivity is a major consideration of plasma etching processes.
In order to obtain the desired contact profile and substrate selectivities, the carbon/fluoride ratio must be optimized. In most cases, the optimum carbon/fluoride ratio for contact etching results in a heavy fluorocarbon polymer deposition process for the dry etch reactor grounded anode. The resulting thick polymer directly effects the particle performance of the process, and consequently, the process stability over time.
The polymer buildup tends to occur around the outer perimeter of the anode outside of the area where the wafer is resting, and this buildup interferes with the etching process. The fluoropolymer tends to become deposited upon cooler surfaces and grounded or non-powered surfaces, such as the anode plate.
When there is a large build up of fluorocarbons in the chamber, there is greater opportunity for contamination of the wafers. The deposition necessitates the removal of the anode plate for cleaning after a given number of wafers typically 500-800 wafers. The removal for cleaning results in equipment downtime for the plasma dry etch machine because of the difficulty in removing and replacing the anode plate. As a result, manual cleaning is very time consuming and greatly hinders the manufacturing process. The polymer film deposition problem is essentially the greatest limiting factor of wafer throughput per chamber clean for most such selective oxide etches. U.S. Pat. No. 4,859,304 entitled "Temperature Controlled Anode for Plasma Dry Etchers for Etching Semiconductor," describes some of the problems associated with fluoropolymer residues.
Presently, dry etch cleaning schemes using oxygen are utilized to handle the problem of fluorocarbon residue removal. Oxygen is currently being used for plasma dry cleaning. The removal of the fluorocarbon polymer is effected by the elemental oxygen produced by plasma ionization. The amount of elemental oxygen produced is proportional to the ionization efficiency of the plasma source. For most of the typical etch tools, the ionization efficiency is on the order of 0.01% to 0.1%.
The present invention increases the amount of elemental oxygen present during the cleaning process, using ozone in lieu of oxygen. Ozone is currently being used for plasma oxide deposition and wet cleaning, processes which are significantly different from the use of ozone for a dry chamber clean. In the presence of sufficient heat, ozone decomposes to form oxygen and a free radical. The decomposition of the ozone could be driven by plasma excitation. The actual weight percentage of ozone injected into the reactor via an ozone generator is approximately 4.2%. Based on this figure, the use of ozone in place of oxygen would result in two to three orders of magnitude more elemental oxygen for reaction with the fluorocarbon polymer residue.